Poly(ethylene terephthalate)polyester with lower diethylene glycol content

ABSTRACT

FORMATION OF CONTAMINANT DIETHYLENE GLYCOL BY-PRODUCT DURING MANUFACTURE OF POLY(ETHYLENE TEREPHTHALATE) PREPOLYMER BY ESTER EXCHANGE OF DIMETHYL TEREPHTHALATE AND ETHYLENE GLYCOL IN THE PRESENCE OF A CATALYST MIXTURE IS MINIMIZED BY INCLUDING LITHIUM ACETATE DIHYDRATE AS AN ESSENTIAL INGREDIENT IN THE CATALYST MIXTURE WHEN THE CATALYST MIXTURE CONSISTS OF ZINC ACETATE DIHYDRATE OR ANTIMONY TRIOXIDE, ALONE OR TOGETHER, AS THE CATALYST OR CATALYSTS FOR SAID MANUFACTURE.

United States Patent Othce 3,749,697 POLY(ETHYLENE TEREPHTHALATE) POLYESTER WITH LOWER DIETHYLENE GLYCOL CONTENT Kenneth T. Barkey, Rochester, N.Y., assignor to Eastman Kodak Company, Rochester, N.Y. No Drawing. Filed Dec. 2, 1971, Ser. No. 204,365 Int. Cl. C10g 17/013 U.S. Cl. 260-75 R 5 Claims ABSTRACT OF THE DISCLOSURE Formation of contaminant diethylene glycol by-product during manufacture of poly(ethylene terephthalate) prepolymer by ester exchange of dimethyl terephthalate and ethylene glycol in the presence of a catalyst mixture is minimized by including lithium acetate dihydrate as an essential ingredient in the catalyst mixture when the catalyst mixture consists of zinc acetate dihydrate or antimony trioxide, alone or together, as the catalyst or catalysts for said manufacture.

FIELD OF THE INVENTION DESCRIPTION OF THE PRIOR ART The preparation of polyesters such as poly(ethylene terephthalate) and poly(propyleneterephthalate) by ester exchange and condensation is well known. U.S. Pats. 2,465,319, 2,727,881 and 3,488,382 and many others describe such processes in great detail. A common feature of a large number of these processes is that they begin with a lower dialkyl ester of a bifunctional dicarboxylic acid which is condensed with a bifunctional glycol, the glycol usually being used as such although it can be used in the form of lower alkanoic acid ester thereof, such esters being equivalents of the glycol. These processes involve the initial preparation, in the presence of a cata lyst, of a substantially monomeric protopolymer under conditions facilitating removal of the lower alkanol formed from the lower dialkyl ester by ester interchange with the glycol. This is sometimes referred to as the first stage or the ester exchange step of the polyester preparation and can be conducted under a variety of conditions using many different types of apparatus. For example, the lower dialkyl ester and the glycol in a mole ratio beginning at about 1 to 1.5 up to about 1 to are placed in a reaction vessel equipped with a packed column with the vessel being heated at a temperature which permits the lower alkanol to pass through the column, with the glycol being retained by the column and returned to the reaction vessel by refluxing, whereby the lower alkanol is theoretically removed during this first stage of the polyester preparation which results in the formation of a monomeric protopolymer.

The catalysts which may be used in the preparation of polyesters according to the above processes include organic and inorganic compounds of metals such as titanium,

3,749,697 Patented July 31, 1973 manganese, antimony, zinc, tin, lead, calcium, cobalt, lithium, combinations thereof, etc., many of which have heretofore been utilized by those skilled in the art. The prior art, for example, discloses a great number of such catalysts, some of which are described in patents such as U.S. Pats. 2,465,319, 2,720,502, 2,727,881, 3,488,382 and others. Specific catalysts heretofore known and which may be utilized include tetraisopropyl titanate, titanium dioxide, zinc acetate, zinc acetyl acetonate, lead oxide, calcium oxide, lithium ethoxide, antimony trioxide, manganese oxides and the like. Generally, the acetates, chlorides, nitrates, sulfates, oxides and alkoxides of one or more of the metals zinc, manganese, tin, lead, titanium, antimony, cobalt and lithium are preferred. For example, the catalyst system of zinc acetate and tetraisopropyl titanate is well suited to attain the desired reaction activity. The catalyst (or catalyst mixture) is generally utilized in a concentration of from about 0.002 percent to about 0.2 percent by Weight of the reactants being condensed. Higher or lower percentages can also be employed. Generally from about 0.001 percent to about 0.05- percent catalyst can be advantageously employed. Preferred ester exchange catalysts include, for example, zinc acetate, manganous acetate, cobaltous acetate, lithium acetate, and tetraisopropyl titanate. However, many others are known. During the first stage manufacture of the above type of monomeric protopolymer polyester, specifically poly- (ethylene terephthalate) prepolymer, by means of catalytic systems using the above described catalysts, a byproduct compound, diethylene glycol (DEG) is formed and reacts With the polyester to form a copolyester which is randomly distributed in the polyester, which itself is a homopolymer, from which copolyester it cannot readily be removed. The copolyester has a lower melting point than does the parent homopolymer polyester and melts over a broader temperature range than does the polyester homopolymer. Furthermore, the copolyester nucleates and crystallizes differently than does the polyester homopolymer. As a result, during subsequent operations involved in the manufacture of a film product from the polyester product containing the copolyester as a contaminant byproduct, many problems arise which have an adverse effect upon the quality and uniformity of the film product made therefrom and upon the processing operations necessarily used in making the film. For example, in the manufacture of the film product, the first stage polyester product is first melted and then cast to form a readily grindable solid product for use in further polymerization steps. Uniform nucleation and rapid crystal growth are essential for casting of such polyester prepolymer in an opaque readily disintegratable solid form. Presence of the copolyester contaminant in excessive and often varying amounts causes the cast polyester prepolymer to solidify inglassy, amorphous form which is difficult to prebreak or crush, grind and fiuidize for the subsequent further polymerization of the polyester prepolymer to a higher degree of polymerization in a second polymerization stage in a known manner. The adverse effects of the copolyester contaminant are carried over into the melt from which the film product ultimately is made when the completely polymerized polyester is extruded. Adverse effects such as increased incidents of tearolfs and breaks are encountered during tentering and drafting operations during which the polyester film is biaxially oriented. Prior to the present invention, the only known way to minimize the latter forms of defects was to increase the inherent viscosity (I.V.) of the individual batches of polyester during the second stage of polymerization, thereby causing longer processing times and consequent loss of production capacity.

3 SUMMARY OF THE INVENTION The present invention comprises the use in the ester exchange step of polyester preparation of poly(ethylene terephth'alate) from dimethyl terephthalate and ethylene glycol of an improved catalyst mixture wherein lithium in the form of a salt of an organic acid, e.g. lithium acetate dihydrate, is used in a minor buffering amount in a two or three part catalyst system with catalytic amounts of zinc, antimony or zinc-antimony catalyst. Preferably, the improved catalyst mixture consists essentially of ilthium: zinczantimony in the ratio by parts per million (p.p.m.) of dimethyl terephthalate (DMT) charged to the reactor of about 20:65 :300, preferably in the forms of lithium acetate dihydrate, zinc acetate dihydrate, and antimony trioxide, respectively. In the latter forms, the amounts of the ingredients used in the catalyst mixture, taken as percents of the weight of DMT charged, will be about 0.03% lithium acetate dihydrate, about 0.02% zinc acetate dihydrate and about 0.035% antimony trioxide, or about 0.12 part lithium acetate dihydrate, 0.08 part zinc acetate dihydrate, and about 0.14 part of antimony trioxide per 388 parts of dimethyl terephthalate charged to the reactor.

The polyester prepolymer formed as a result of the practice of the present invention is characterized in that the amount of diethylene glycol present in the first stage polyester product is advantageously lower than that present in corresponding prior art product, and the melting point of the first stage polyester prepolymer product is significantly higher than that of the corresponding prior art product, using in each prior case a catalyst mixture consisting of zinc and/or antimony compounds. The polyester prepolymer product is characterized in that it has advantageously improved casting, grinding, and fluidization properties for the further manufacture of poly(ethylene terephthalate) powder, which powder subsequently is melt extruded into film form, drafted and tentered to form a biaxially-oriented poly(ethylene terephthalate) film support in a known way for use in manufacture of photographic film products.

The following examples illustrate the advantages of a preferred embodiment of the invention over the prior art practice.

i 2 moles. i 4 moles. Based on weight of dimethyl terephthalate charged to reactor.

TABLE II.PROCESS DATA AND PROPERTIES OF POLY (ETHYLENE TEREPHTHALATE) PREPOLYMER Example- 1 2 Time to complete ester exchange reaction to form prepolymer, min 44 26 Inherent viscosity of prepolymen- 0. 44 0. 43 Carboxyl radical content, meq./kg. 36 14 Melting point, C 256 260-261 Diethylcnc glycol content, mole percent of prepolymer" 3. 1. 5

Thus, it is evident from comparison of the process data and properties of poly(ethylene terephthalate) prepolymers (Table II) made by the prior art method (Example 1) in the absence of lithium acetate dihydrate in the catalyst mixture and the method of the present invention (Example 2) wherein lithium acetate dihydrate is present in the catalyst mixture that the use of only a minor amount of lithium acetate dihydrate (Table 1, Example 2) has reduced the reaction time for the ester exchange reaction by 18 minutes, from 44 minutes to 26 minutes. Also, while the inherent viscosity of the prepolymer polyester has not been significantly affected, i.e. 0.44 and 0.43, the carboxyl radical content, which under normal circumstances is indicative of presence of diethylene glycol, has been reduced greatly, from 36 to 14 mole equivalent (meq.) per kilogram (kg) of prepolymer polyester. Further, the melting point of the poly(ethylene terephthalate) prepolymer (prepolymer polyester) has advantageously been raised from 256 C. to 260-261 C. by practice of the present invention. And, finally, the actual amount of diethylene glycol present in said prepolymer polyester has been reduced by two full percentage points, i.e. from 3.5 mole percent to 1.5 mole percent in the prepolymer polyester. In practice of the invention, it has been found that lowering of the mole percent of diethylene glycol present by one mole percent results in an increase in melting point of the polyester prepolymer by 2.15 C. In the present example, a reduction of 2 mole percent in diethylene glycol content of the polyester prepolymer has resulted in a 4 to 5 C. increase in melting point. The prepolymer polyester thus has physical property characteristics which one skilled in the art of polycondensing the polyester prepolymer to the polyester and the manufacture of the so-obtained poly (ethylene terephthalate) into film form by known prior art methods will readily recognize as advantageous for the forming of biaxially-oriented film therefrom.

Procedures for carrying out the polycondensation process and procedures for forming biaxially-oriented film from poly(ethylene terephthalate) are described, for example, in US. Pats. 3,043,564 and 3,048,564, and such procedures may be used to make the polyester and to make biaxiallyoriented film from the poly(ethylene terephthalate) of the present invention. When the poly(ethylene terephthalate) of the present invention is being tentered and drawn for the making into film, the adverse effects discussed above, e.g. tearoffs and breaks, are found to be absent or considerably reduced in frequency.

While the amount of lithium used in practice of the invention has been shown in Example 2 to be 21 p.p.m. of lithium as Li (equivalent to 0.1176 g. of lithium acetate dihydrate) in a reaction mixture of two gram moles of dimethyl terephthalate with four gram moles of ethylene glycol, resulting in the formation of two gram moles of poly(ethylene terephthalate) prepolymer, the advantageous results of the invention also may be obtained by the use of from about 15 to about 60 p.p.m. of lithium as Li (equivalent to about 0.078 g. to about 0.340 g. of lithium acetate dihydrate) in a reaction mixture of two gram moles (388 g.) of dimethyl terephthalate with four gram moles (248 g.) of ethylene glycol. The amounts of antimony catalyst then used may range from about p.p.m. to about 500 p.p.m. (equivalent to from about 0.046 g. to about 0.230 g. of antimony trioxide) in the same reaction mixture. The amounts of zinc catalyst then used may range from about 32 p.p.m. to about p.p.m. (equivalent to from about 0.042 g. to about 0.168 g. of zinc acetate dihydrate) in the same reaction mixture.

Preferably, the improved catalyst mixture of this invention will consist of 65 p.p.m. of zinc, 300 p.p.m. of antimony and from 21 to 35 p.p.m. of lithium in the form of the equivalent amounts of zinc acetate dihydrate, antimony trioxide and lithium acetate dihydrate, respectively, based on the weight of dimethyl terephthalate charged to the reactor.

Example 3 The advantageous effect of the use of a minor amount of lithium in the zinc-antimony catalyst mixture has been found to be obtained also when recycled ethylene glycol from the poly(ethylene terephthalate) prepolymer recovery system is used for manufacture of more of the polyester prepolymer with a new supply of dimethyl terephthalate.

Recycled ethylene glycol recovered from a previous poly(ethylene terephthalate) prepolymer reaction mass is used in the ester exchange reaction in a subsequent batch. Water (2-4 percent) is a major contaminant of such ethylene glycol. A high melting point for the polyester pre polymer is indicative of low diethylene glycol content in the prepolymer. The melting point raising effect of the use of lithium (as lithium acetate dihydrate) in the catalyst system for the preparation of poly(ethylene terephthalate) prepolymer from recycled ethylene glycol along 5 cover is shown b the followin data. w1th new d1methylene terephthalate 1s shown 1n the fol- Scrip poly(ethysgene terephthilate) from film mamb lowmg laboratory batches of Table facturing operations was subjected to glycolysis reactions TABLE III to obtain polyester of lower molecular weight for reproc- Ppm catalyst prepolymer essing to film grade poly(ethylene terephthalate). Various n e g catalyst systems were used to accelerate the breakdown of Batch L13 Scoslty pomt' the polyester and to minimize diethylene glycol formagg 28g 2 8-2 322 tion. The scrap poly(ethylene terephthalate) had the fol- 65 300 0 1 lowing characteristics before glycolysis: 1 I 65 300 21 0. 42 259 TABLE V =$355233iiiiifiiifi figefiief Inherent viscosity (I.V.) 0.65 Chargedas lithium acetate dihydrate. carboxyl (COOH), meqjkg 3 5 From the data it is seen that the use of Li (lithium ace- Melting point, C. 257 tats dihydrate) gave poly(ethylene terephthalate) In each of the followin batches A-D 300 g of oly(etho a :1 y a polyme.r meltmg 2 higher m each batch (1b andhzb) ylene terephthalate) were reacted with 450 g. of ethylene W e It was Present as compared to im patches 2 ag glycol (1.5 weight ratio of ethylene glycol to scrap poly hthmm was absqnt (1a and and 1S mdlcatlve o i (ethylene terephthalate)) at the boiling point (198-205 fact that less. methylene glycol was formed even wlt 0). Times necessary to give clear solutions varied de- Water present In the glycol pending upon the catalyst system and its concentration. The advantageous efiects of i presence of hthlum m After the scrap had been dissolved and reacted to low the .catalysi mlxture for regucmg the Content of conmolecular weight oligomers, excess ethylene glycol was tammant methylene glycol m the .Prepolymer .poiyesgr distilled off and the melt was polymerized for two hours have .also f to F obtamed when hthlum i at 260 C. and at a pressure of less than 100 microns used c9mbmatlon Zmc as only Otiher .cata1y.t1c (0.1 mm. Hg). The resulting poly(ethylene terephthalate) maienal m the catalyst system m colilbniauon Wlth 3O prepolymer was cast and analyzed. The resulting data are ant1mony as the only other catalytic mater1al 1n the cata- Shown in Table VI lyst system. Examples 4 and 5 further illustrate such advantageous results obtained by practice of the invention. TABLE W In the examples it is to be noted again that the obtain- P.p.m. After two hrsofmelt ment of a higher melting point for the prepolymer poly- Catalyst Polymenmwn ester is indicative of the lowering of the diethylene glycol Batch Glycolysls, M.P., content in the polyesten No. Z111 L12 mmutes I.V. COOH 0. Example 4 16% 3 2 0. 20, 8: i; 13; i Distilled monomer (250 g.) (i.e. poly(ethylene tereph- 3 3: 3% 3i; thalate) prepolymer) separated from poly(ethylene ter- 1Chargedaszincacetatedjhydmte. ephthalate) prepolymer 1n a recovery st1ll and contamzcharged as lithium acetate dihydmte ing 280 p.p.m. of zinc and 150 p.p.m. of antimony catalysts (based on weight of monomer charged) was reacted T0 ellmlnate the efiect 10W 111016611131 Weight, 38 additionally with new dimethyl terephthalate (388 g.) evidenced by the inherent viscosity (I.V.) of the prepolyand additional new ethylene glycol. In batch a, 300 p.p.m. r a its melting Point, batches B and D Were P y of Sb were added (as Sb 0 in batch b, 21 p.p.m. of Li erized further by solid phase buildup at 210 C. for seven (charged as lithium acetate dihydrate) and 300 p.p.m. of hours. The results obtained are shown in Table VII.

TABLE VII P.p.1:n. catalyst 0 hour 1 hour 2 hours 5 hours 7 hours COOH, COOH, COOH, COOH, COOH, M.P.; Batch N0. Zn Li I.V. meq./kg. I.V. meqJkg. I.V. meq./kg. I.V. meq./kg. I.V. meq./kg. C.

1 Charged as zinc acetate dihydrate. 2 Charged as lithium acetate dihydrate.

Sb (as Sb O were added. The results obtained are shown From the data in Table VII, it is to be noted that in Table IV. after polymerization for 7 hours in each batch, batch TABLE IV D, which had 5; p.p.m. of lithium present along with 162 Pro fiesof p.p.lmci of z11n(c lfil 1the cattalys;1 llilliitlllf, resuclited inha re- P cyce poy et yene erep t aate pro uct aving '%ef?e'1, prepolymer COOH content of 38 meq./kg., compared to original Batch No. n 1 5 9 95 2 CO'OI-I content of 35 meq./kg. in the scrap polyester and a melt1ng point of 253 compared to the original of 257 g 9, 3 ggg c. The results from batch B show that in the eeeenee of lithium, the zinc catalyst alone gave polyester prod-uct llnhreent which had 53 meq./kg. of COOH (compared to 38 forv From the data, it is to be noted that inclusion of lithium d a me Point Of y 247, 111016 Percent in the catalyst mixture again caused lesser amounts of di- EG) compared to 111016 P rcent DEG). ethylene glycol to form (higher melting point) and lesser Therefore, it is evident that use of lithium acetate dihypolyester acidity as evidenced by lower carboxyl content drate in the catalyst mixture is advantageous for th (15 meq./kg. of COOH versus 29 meq./kg. of COOH) Scrap polyester recovery process as well as for the manufor the poly(ethylene terephthalate) prepolyrner. facture of fresh poly(ethylene terephthalate) prepolymer.

Example 5 The recovery of waste poly(ethylene terephthalate) from film manufacturing operations is an ecological and economical necessity. Practice of the invention in such re- The recycled prepolymer is in a form suitable for polycondensation to a poly(ethylene terephthalate) which can be made into a useful film product.

As used throughout this description, it is to be understood that the inherent viscosity (I.V.) of poly(ethylene terephthalate) prepolymer is in the range of about 0.40 to 0.44 and the inherent viscosity of polycondensed polyester, i.e. poly(ethylene terephthalate) is in the range of about 0.60 to 0.65.

Although the nature of the activity of the lithium acetate dihydrate in inhibiting the formation of diethylene glycol in accordance with the invention is not definitely known, it is believed that the acidity of the reaction mass is in some manner affected. However, when experiments similar to those given in the examples were carried out using sodium acetate as a substitute for the lithium acetate in the catalyst mixture of the invention the results obtained were not as good and, in addition, the biaxiallyoriented films made from the sodium acetate experiments were found to be hazy and not clear as were those obtained when lithium acetate dihydrate Was used in the catalyst mixtures of the invention.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.

I claim:

1. In a process for manufacturing a polyester prepolymer of dimethyl terephthalate and ethylene glycol by an ester exchange process which process comprises heating in a reactor a liquid mixture of said ethylene glycol and said dimethyl terephthalate in the presence of an ester exchange catalyst mixture at an elevated temperature for a period of time sufficient to form poly(ethylene terephthalate) prepolymer and in which process diethylene glycol forms as a contaminant by-product material, the improvement which comprises carrying out said heating in the presence of an ester exchange catalyst mixture consisting of (a) a catalytic amount of at least one catalyst selected from zinc acetate dihydrate and antimony trioxide, and (b) lithium acetate dihydrate in an amount essentially suflicient to inhibit formation of said contaminant diethylene glycol by-product material.

2. In a process for manufacturing a polyester prepolymer of dimethyl terephthalate and ethylene glycol by an ester exchange process which process comprises heating in a reactor a liquid mixture of said ethylene glycol and said dimethyl terephthalate in the presence of an ester exchange catalyst mixture at an elevated temperature for a period of time sufficient to form poly(ethylene terephthalate) and in which process diethylene glycol forms as a contaminant by-product material, the improvement which comprises carrying out said heating in the presence of a catalyst mixture selected from (a) a catalyst mixture of zinc and lithium charged to the reactor in the forms of their acetate dihydrates, (b) a catalyst mixture of antimony and lithium charged to the reactor in the forms of antimony trioxide and lithium acetate dihydrate, respectively, and (c) a catalyst mixture of antimony, zinc and lithium charged to the reactor in the forms of antimony trioxide, zinc acetate dihydrate and lithium acetate dihydrate, respectively, each of said catalyst mixtures essentially containing lithium charged as lithium acetate dihydrate as an essential ingredient for inhibiting the formation of diethylene glycol in the poly(ethylene terephthalate) prepolymer formed in said process.

3. The process according to claim 2 wherein the catalyst mixture consists essentially of a mixture of (a) from 15 to about p.p.m. of lithium, (b) from 0 to about 500 p.p.m. of antimony, and (c) from 0 to about 130 p.p.m. of zinc based on the weight of dimethyl terephthalate charged to the reactor.

4. The process according to claim 2 wherein the catalyst mixture consists essentially of from about 15 to about 60 p.p.m. of lithium and from about I to 500 p.p.m. of antimony.

5. The process according to claim 2 wherein the catalyst mixture consists essentially of from about 15 to about 60 p.p.m. of lithium and from about 32 to about p.p.m. of zinc.

References Cited UNITED STATES PATENTS 3,428,587 2/1969 Piirma 26022 MELVIN GOLDSTEIN, Primary Examiner US. Cl. X.R. 

